1. Field of the Invention
The present invention relates to methods for treating subterranean wells using improved borate cross-linking compositions and cross-linked aqueous well treating fluids.
2. Description of the Prior Art
High viscosity aqueous cross-linked gels are used in a variety of operations and treatments carried out in oil and gas wells. Such operations and treatments include, but are not limited to, well completion operations, fluid loss control treatments, production stimulation treatments, formation permeability conformance operations and treatments to reduce water production.
An example of a production stimulation treatment utilizing a high viscosity cross-linked gelled fluid is hydraulic fracturing. In hydraulic fracturing treatments, the high viscosity fluid is utilized as a fracturing fluid and also carries particulate propping agents, e.g., sand, into the fractures formed. That is, the fracturing fluid is pumped through the well bore into a formation to be stimulated at a rate and pressure such that fractures are formed and extended in the formation. Propping agent is suspended in the fracturing fluid so that it is deposited in the fractures when the gel is broken and returned to the surface. The propping agent functions to prevent the formed fractures from closing whereby conductive channels are formed through which produced fluids can flow to the well bore.
Borate ion has long been used as a cross-linking agent for forming high viscosity cross-linked gelled aqueous well treating fluids. Various sources of borate have been utilized including boric acid, borax, sodium tetraborate, slightly water soluble borates such as ulexite, and other proprietary compositions comprised of boric acid and dimers and trimers of borate ions. These solid materials which form or contain borate ion have varying solubilities in water and can cause operational problems when used as cross-linking agents in the preparation of high viscosity cross-linked gelled aqueous well treating fluids. For example, when the weather conditions are damp or wet, the solids tend to clump whereby they are difficult to meter into a gelled solution. While the solid materials are soluble in water, it is generally difficult to prepare a high concentration solution of the materials. When high concentration solutions are prepared and used, large volumes are generally required. Also, in cold weather, the high concentration solutions crystalize making pumping and metering difficult.
Another problem involved in the preparation of a high viscosity borate ion cross-linked gelled aqueous treating fluid has been the necessity of maintaining the pH of the fluid at a high level by the addition of a caustic solution thereto. That is, the pH of the fluid controls the equilibrium between boric acid and borate ion, the borate ion being the boron species which causes gelled aqueous fluids to cross-link. When the temperature of the fluid increases, the pH of the fluid decreases. Thus, the heretofore used borate cross-linked well treating fluids have been difficult to prepare as a result of temperature and/or pH changes. By elevating the pH of the gelled fluid, the borate ion concentration in the fluid is higher. At relatively high temperatures, the pH of the treating fluid must be very high in order to allow the treating fluid to be cross-linked using a moderate amount of the borate ion source. However, when the water utilized for forming the gelled fluid contains salts such as calcium and magnesium salts which are present in brines and sea water, calcium and magnesium salts are precipitated as the pH of the fluid is raised to the high level required to minimize the quantity of borate source necessary for cross-linking. The precipitation of the salts causes even greater amounts of caustic solution to be required. The use of caustic solution to raise the pH of a borate ion cross-linked well treating fluid and the necessity of including excess borate ion in the fluid to insure stability increases the cost of the fluid and the well treatment performed using the fluid. Thus, there is a need for improved liquid borate cross-linking compositions which can be utilized to form stable borate cross-linked gelled aqueous well treating fluids and methods of using such fluids.